This paper describes the technical details of SemRetriev, a prototype system for image retrieval which combines the use of an ontology which structures an image repository and of CBIR techniques. The system models a real-world situation by including pictures gathered from the Internet and is designed for exploratory picture search. SemRetriev proposes two methods for retrieving images, using keywords and visual similarity and the ontology is useful in both cases. First, it is employed to reformulate queries and to render propose structured picture sets in response and second, it is used to propose CBIR processes in different subsets of the conceptual hierarchy. The user supplies a query term and the system furnishes images corresponding to the subconcepts of the queried term. It is then possible to narrow or extend the current search, to see detailed image sets or detailed images. It is equally possible to click any displayed image and obtain visually similar answers. The first experi...